In some vehicles, such as boats, automobiles, motorcycles, etc., speedometers are driven mechanically by a cable connected to a vehicle's transmission. When the vehicle is in motion, a speedometer gear assembly turns a speedometer cable, which then turns the speedometer mechanism itself. Typically, a small permanent magnet affixed to the speedometer cable interacts with a small aluminum cup or “speedcup” attached to the shaft of the pointer on the speedometer instrument. As the magnet rotates near the cup, the changing magnetic field produces eddy currents in the cup, which themselves produce another magnetic field. The effect is that the magnet exerts a torque on the cup, “dragging” it, and thus the speedometer pointer, in the direction of its rotation with no mechanical connection between them.
The pointer shaft is held toward zero by a fine torsion spring. The torque on the cup increases with the speed of rotation of the magnet. Thus an increase in the speed of the vehicle will twist the cup and speedometer pointer against the spring. The cup and pointer will turn until the torque of the eddy currents on the cup is balanced by the opposing torque of the spring, and then stop. Given the torque on the cup is proportional to the vehicle's speed, and the spring's deflection is proportional to the torque, the angle of the pointer is also proportional to the speed, so that equally spaced markers on the dial can be used for gaps in speed. At a given speed the pointer will remain motionless and pointing to the appropriate number on the speedometer's dial.
The return spring is calibrated such that a given revolution speed per minute (rpm) of the cable corresponds to a specific speed indication on the speedometer. This calibration must take into account several factors, examples include: ratios of the tailshaft gears that drive the flexible cable, the final drive ratio in the differential, and the diameter, pressure, and temperature of the driven tires.
Over time, the accuracy of these speedometers can degrade. In addition, varying tire sizes, changing drive system gear ratios, and aging mechanical systems can affect the accuracy of the speedometer, as many are factors taken in account when calibrating. A driver may be unaware of the inaccuracy of the speedometer and may drive the vehicle faster than what the driver thought due to reliance on the inaccurate speedometer reading. This can result in expensive and unnecessary speeding violations.
As may be appreciated, speedometer readings also affect the odometer. In instances where a speedometer is reading higher than it should, the odometer may be registering higher mileage than the vehicle has actually traveled. Higher odometer readings may decrease the value on a vehicle thus costing the owner sunk costs from the accelerated depreciated value in a sale of the vehicle.
Solutions such as recalibrating the speedometer or adjusting gear ratios are vehicle dependent and require significant time, effort, and expertise. These solutions run the risk of disrupting or harming other components in the engine and can potentially disrupt the current speedometer system. Even so, these solutions can become obsolete as soon as a relevant change in the system is made. Furthermore, adjusting gear ratios is not an exact science, thereby resulting in only a close correction of the speedometer reading.
A need therefore exists for a method and system to provide a speedometer reading for a mechanically driven speedometer that sustains an accurate speedometer performance longer than the prior art.